<!DOCTYPE html>
<html lang="zh">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .main section h1,
        .main article h2 {
            font-weight: bold;
        }

        .main aside h3 {
            font-style: italic;
        }
    </style>
</head>

<body>
    <div class="main">
        <section id="intro">
            <h1>Welcome</h1>
            <p>This is the introduction section.</p>
        </section>
        <article class="post">
            <h2>Post Title</h2>
            <p>Post content...</p>
            <ul>
                <li>Item 1</li>
                <li>Item 2</li>
            </ul>
        </article>
        <aside class="sidebar">
            <h3>Sidebar Title</h3>
            <p>Sidebar content...</p>
        </aside>
    </div>
</body>

</html>